1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Related Art
In the automatic transmission of the prior art, the rotation output from the engine is transmitted through a fluid transmission unit, such as a torque converter, to a speed change unit. Between the fluid transmission unit and the speed change unit is arranged a first clutch (or input clutch) which can be applied/released for shifting between a neutral range (hereinafter "N-range") and a forward running range (hereinafter "D-range").
In the prior art automatic transmission, when the D-range is selected with the vehicle speed at zero, with the accelerator pedal released and the brake pedal depressed, neutral control is effected by lowering the oil pressure of the hydraulic servo to release the first clutch so that the load on the engine is reduced to improve the fuel economy and to prevent the vehicle from vibrating.
In the automatic transmission thus constructed, when the first clutch is to be applied, the output RPM of the torque converter is detected so that application shock due to engagement of the first clutch may be prevented by feeding, to the hydraulic servo of the first clutch, an oil pressure corresponding to the output RPM, as disclosed in Japanese Patent Laid-Open No. 136055/1986.
In the automatic transmission of the prior art described above, however, the output RPM of the torque converter is detected, and the oil pressure is generated, according to the detected output RPM so that proper engaging characteristics cannot be attained.
Generally speaking, when the RPM of a rotary member is to be detected, a plurality of projections, formed on the rotary member, are detected by an RPM detecting sensor so that the RPM is calculated in terms of the time period from the detection of one projection to the detection of a subsequent projection.
If the output RPM of the torque converter is to be detected by the RPM detecting sensor when the vehicle is standing still as in neutral control, the time period between detection of one projection and detection of the next one is prolonged because of the relatively small output RPM, so that the time period required for calculating the output RPM is likewise prolonged and detection is thereby delayed.
Thus, when the vehicle stops, poor responsiveness makes real time feedback control impossible which, in turn, makes it impossible to achieve the proper engaging characteristics.
It is theoretically possible to correct and learn the oil pressure for applying the first clutch the next time, if the RPM change rate of the output RPM, as detected, is different from a target RPM change rate. However, this learning control encounters several problems.
Specifically, at the initial stage of engagement of the first clutch, the output RPM is not only unstable because of the delay in the rise of the oil pressure fed to the hydraulic servo but also fluctuates due to the influence of factors other than the oil pressure.
While the output RPM is relatively low, moreover, it cannot be detected because the RPM detecting sensor has a detection limit.
Thus, not only does the output RPM fluctuate due to various factors but, also, the output RPM cannot be detected, so that an accurate RPM change rate cannot be determined, thus inviting erroneous learning.